Off the southern coast of Kanto there lies an archipelago, a chain of seven large islands interspersed with handful of more remote ones, dubbed the Sevii Islands. These islands have never been more than sparsely populated, but the population was driven to the mainland or other regions as a consequence of the Disasters. The Sevii Islands suffered a particularly severe beating from the floods, storms and earthquakes that also shook Kanjohto, but unlike the mainland, the islands did not suffer massive ecological shifts. In fact, since the Disasters, the islands have become a sanctuary for a number of Pokémon, including those fleeing the changing conditions in Kanjohto. The Disasters also brought about migrations from other, more distant regions, drawing unique Pokémon never-before-seen in this part of the world. Along with it, they brought a mysterious power which now seems to radiate from the islands themselves. This influence has caused some familiar Pokémon to move to conditions for which they are normally unsuited, causing them to adapt rapidly in before unseen ways. Furthermore, the power has caused some Pokémon native to the Kanto region to evolve in new and unique ways not achieved on the mainland.

Although now largely abandoned, there are a few humans that have began to return to the islands since the Disasters, to study these mysterious changes or to explore the wilds. Trainers have also been recently returning to the island with the service by small ferries making regular trips to the islands, though not many have come to stay. This resurgence has been mostly due to one of the islands' few permanent residents. Lani, a native of the distant region of Alola, has renewed interest in the abandoned islands, spreading the word about the new Pokémon, as well as instituting a tradition from her home region adapted for the islands to offer a challenge to trainers and draw interest to the islands.

The Island Challenge, a long-held tradition and precursor to the Pokémon League in Alola, has been implemented in a more limited capacity across the nine major islands, and while it lacks the captains and kahunas of Alola, Lani has established a number of challenges, even more than those traditionally found in Alola. Lani has implemented 18 challenges, one per each known Pokémon type, working with the Gym Leaders of Kanjohto to construct gauntlets that characterize every type. Successfully completing these challenges will entitle the trainers to prizes, including the mysterious Z Crystal and access to the power of Z Moves.